1. Field of the Invention
The present invention relates to a condenser microphone and, more particularly, is effective for a condenser microphone having a form in which a microphone unit section incorporates an impedance converter, and a circuit storage section incorporating a low cut circuit, an output circuit, etc. and the microphone unit section are connected by a dedicated cable.
2. Related Background of the Invention
Since an impedance of a microphone unit for converting voice into electric signals is high, an impedance converter configured mainly by a FET (Field Effect Transistor) is used in a condenser microphone. Further, in order to enhance performance as a microphone, a low cut circuit, an output circuit, etc., are incorporated in the microphone.
Microphones are manufactured in a variety of forms according to their uses. In a use in which the presence of a microphone is inconspicuous, it does not bring any problem even if the microphone has a certain volume, therefore, it is possible to incorporate circuit sections such as the impedance converter, the low cut circuit, the output circuit, etc., described above in the microphone case. However, a microphone for conference, a microphone for a church choir, a tiepin-type microphone, etc., are needed to be reduced in size so that the microphone itself is inconspicuous, therefore, it is not possible to incorporate all of the circuit sections in the microphone case. Because of this, for the microphone for conference, the microphone for a church choir, the tiepin-type microphone, etc., (hereinafter, these are referred to as “compact microphones”), a form is employed in which the microphone unit section incorporates the impedance converter, and a circuit storage section incorporating the low cut circuit, the output circuit, etc. and the microphone unit section are connected by a dedicated cable. The voice signal of the microphone is output from the output circuit incorporated in the circuit storage section. A section including the microphone unit and impedance converter is referred to as a “microphone unit section” and the circuit storage section that incorporates the low cut circuit and output circuit is referred to as a “power module section”, therefore, the terms “microphone unit section” and “power module section” are used in this specification hereinafter.
The dedicated cable that connects the microphone unit section and power module section is composed of a two-core signal wire and a shielding wire for covering and shielding the signal wire. More specifically, the dedicated line is composed of a power line for supplying power to the microphone unit section, a signal line for transferring a voice signal output from the impedance converter to the power module section, and a shield line for electrostatically shielding and grounding the power line and the signal line. The dedicated cable transmits a voice signal in an unbalanced mode, therefore, may suffer from external noises. If an electromagnetic wave penetrates the dedicated cable from the outside, the electromagnetic wave enters into the microphone unit section and the power module section and it is detected by a semiconductor used in the microphone unit section and the microphone unit section, and noises are produced.
Recently, a mobile phone prevails widely and the situation in which a mobile phone is used in the vicinity of a microphone becomes more likely to occur, and there arises a serious problem in that noises occur in a microphone due to the influence of the high frequency electromagnetic waves used by the mobile phone. In particular, in the case of the compact microphone, inter alia, in the microphone for a choir, the microphone unit section is suspended from the ceiling of a church etc. and the power module section is attached on the upper side of the ceiling plate in order to prevent the appearance of the ceiling from being degraded, therefore, noises due to electromagnetic waves are becoming a problem. FIG. 3 shows an example of such a suspended type microphone. In FIG. 3, a hole for attaching a power module section 2 is drilled in a ceiling 7 of a church etc. and the power module section 2 is buried in the hole with its outer circumferential surface covered with a shield cover 3. The hole of the ceiling plate is blocked up with a panel 4. The panel 4 has a role to make the attached power module section 2 more inconspicuous and to shield the power module section 2. Penetrating through the panel 4, a flexible pipe 5 hangs and at the lower end of the flexible pipe 5, a microphone unit section 1 is attached. To the microphone unit section 1, a dedicated cable is connected as described above, and the dedicated cable runs through the flexible pipe 5 and the respective lines constituting the dedicated cable are connected to a predetermined terminal of the power module section 2. From the power module section 2, a cord 6 is led out on the upper side of the ceiling 7 so that a voice signal enters an external circuit.
Conventional examples of the power module section 2 are shown in FIG. 4 and FIG. 5. The power module section 2 is composed mainly of a substrate 10, a shield case 3, an input side screw terminal 11, and an output side screw terminal 12. On the substrate 10, a circuit section including the low cut circuit and the output circuit is formed, as described above, and it is so designed that a voice signal from the microphone unit section is input to the circuit section via the screw terminal 11 and a plurality of wiring patterns 13 electrically connected to each terminal of the screw terminal 11. To the wiring pattern 13, a high frequency noise countermeasure component 14 composed of a capacitor etc. is connected; noise coming from the outside. The configuration of the output side of the circuit section is substantially the same as that of the input side and a plurality of wiring patterns 15 for outputting a voice signal are electrically connected to the respective terminals of the output side screw terminal 12. To the wiring pattern 15, a high frequency noise countermeasure component 16 is connected. The high frequency noise countermeasure component 14 is arranged at the side of the screw terminal 11 in order to prevent physical interference with the screw terminal 11 and the high frequency noise countermeasure component 16 is arranged at the side of the screw terminal 12 in order to prevent physical interference with the screw terminal 12, respectively, and in accordance with this, part of the wiring patterns 13 and 15 is formed so as to take a circuitous route between the screw terminals 11 and 12 and the circuit section.
The reason that a screw terminal is used in the power module section 2, in particular, the reason that the input side screw terminal 11 is used in the power module section 2 is that it is necessary to adjust the length of the dedicated cable that connects the microphone unit section and the power module section 2. In other words, the length of the dedicated cable differs depending on various conditions such as the height of the ceiling of a building at which a microphone is installed and the dedicated cable is cut into a length suited to the condition for use. Because of this, if the end portion of the dedicated cable is directly connected to the circuit substrate 10 of the power module section 2 by soldering, wiring work is troublesome; therefore, a screw terminal capable of wiring by screwing is used. Since the situation is the same on the output side of the power module section 2, the screw terminal 12 is also used on the output side.
The dimension of the screw terminals 11 and 12 becomes large since a microphone cord is connected by means of mechanical screwing. In addition to this, there exist the plural wiring patterns 13 and 15 exposed without being electrostatically shielded between the circuit section electrostatically shielded by the shield case 3 and the screw terminals 11 and 12. Part of the plural exposed wiring patterns 11 and 12 is formed in a circuitous manner and increased in length as described above, therefore, unprotected against the electromagnetic waves from the outside and it is more likely that a high frequency current enters therethrough. Further, the core wire part exposed from the shield wire of the input/output microphone cord is screwed onto the screw terminals 11 and 12 of the input/output section; therefore, the core wire part exposed from the shield wire is also unprotected against the electromagnetic waves from the outside. Thus, it is configured such that electromagnetic waves are likely to penetrate through the core wire exposed from the shield wire or through the wiring patterns 13 and 15. If electromagnetic waves penetrate through the exposed core wire or the wiring patterns 13 and 15, the electromagnetic waves enter into the shield case 3 of the power module section 2 and as a result, noises are mixed in a voice signal. Although the high frequency noise countermeasure components 16 are connected to the screw terminals 11 and 12, if electromagnetic waves once penetrate through the core wire or the wiring patterns 13 and 15, it is not possible to prevent the electromagnetic waves from causing noises.
Incidentally, in the conventional environment in which electromagnetic waves do not come and go so frequently, the conventional configuration of the power module section 2 has not brought about any problem. This is because it is unlikely that electromagnetic waves penetrate from the outside. However, in such a recent environment in which the individual carries his/her own mobile phone, electromagnetic waves exist everywhere and now the situation is that the influence of the electromagnetic waves on the microphone cannot be ignored. It has been found out that in a condenser microphone in a form shown in FIG. 3, even if the power module section 2 shown in FIG. 4 and FIG. 5 is used, the electromagnetic waves having penetrated through the cable connecting the microphone unit 1 and the power module section 2 enter into the shield case 3 through the wiring patterns 13 and 15 and produce noises. Therefore, countermeasures against noises due to electromagnetic waves are desired in a condenser microphone in a form in which a microphone unit section and a power module section are separated and connected with a cable.
Conventionally, various countermeasures against noises due to electromagnetic waves in a condenser microphone have been proposed. For example, a condenser microphone has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 2004-7156), in which in order to not only electrically stabilize the contact between the condenser microphone main body and a substrate but also reduce the high frequency noise level, in addition to the condenser microphone main body, a spring terminal for outputting a voice signal of the condenser microphone main body and an insulator for causing one end of the spring terminal to come into contact with a contact of the substrate of the condenser microphone main body by attaching the spring terminal are comprised, and the other end of the spring terminal is made into a spiral shape.
Further, an electret condenser microphone has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 2003-230195), in which an electroacoustic transducing section is constituted by charging the inner surface of a front end wall of a capsule and disposing a vibration diaphragm at a position in opposition to the front end wall, the capsule is blocked by engaging a substrate with the rear end portion of the capsule, a FET for transducing the acoustic vibration of the vibration diaphragm into an electric signal is provided to the substrate, and a capacitor and a varistor are further provided in parallel between the FET and a ground terminal.
The respective inventions described in Japanese Patent Application Laid-Open No. 2004-7156 and Japanese Patent Application Laid-Open No. 2003-230195 relate to a microphone of a type incorporated into, for example, a mobile phone and are based on the idea that high frequency noise countermeasure components are additionally provided. In addition, the invention described in Japanese Patent Application Laid-Open No. 2003-230195 is one that the microphone unit section is stored in the capsule and shielded.
However, none of the inventions described in Japanese Patent Application Laid-Open No. 2004-7156 and Japanese Patent Application Laid-Open No. 2003-230195 relate to the high frequency noise countermeasures in a condenser microphone in a form in which a microphone unit section and a power module section are separated and connected by a dedicated cable.